According to data sent back by NASA’s MESSENGER spacecraft, the planet Mercury is now 8.6 miles smaller in diameter than it was a few billion years ago. But it’s not shrinking because of the sun’s heat -it’s actually getting smaller because it’s cooling down.

All planets are chilling and shedding heat to varying degrees, and Mercury is no exception, despite being the closest planet to the sun. But the process has taken an unusual toll on Mercury's already cratered complexion, forming cliffs that can soar up to two miles (three kilometers) high—about as tall as Mount St. Helens—and long chains of aligned ridges that snake across the face of the planet for up to 1,050 miles (1,700 kilometers), more than twice the length of Florida.

Mapping this terrain on foot will never be an option, since surface temperatures can reach a toasty 800 degrees Fahrenheit (430 degrees Celsius). Scientists instead used observations from NASA's sun-shielded MESSENGER spacecraft, which has been orbiting Mercury since 2011.